Exhaust-driven turbochargers are typically provided for internal combustion engines in order to improve their performance by increasing the pressure, temperature and density of air in an engine inlet manifold which is used to supply a fuel/air mixture to the engine cylinders. This makes it possible to burn more fuel in each cycle of the engine, thereby increasing the power output. Exhaust-driven turbochargers make use of energy contained in the engine exhaust gases which would otherwise go to waste, and therefore can contribute to an improvement in the overall efficiency of the engine. However, single-stage turbochargers are not particularly effective at the low rates of exhaust gas flow which are associated with low engine operating speeds. This leads to the problem of ‘turbo lag’, i.e. a noticeable delay between a driver demanding power and the engine delivering the required response, because the engine speed needs to rise to a certain level before the turbocharger operates effectively. These problems can be ameliorated to some extent by the use of multi-stage turbochargers.
An alternative solution is to provide a supercharger in addition to the turbocharger. The supercharger is driven mechanically by the engine, to perform the same task of raising the pressure, temperature and density of air in the inlet manifold when the engine is running at low speeds such that the turbocharger is ineffective. A supercharger is engaged at the instant of a driver demanding power to substantially increase the volume of air admitted to the engine. With appropriate fuelling, a supercharger can eliminate turbo lag. An additional benefit is that the immediate increase in air flow allows the turbocharger impeller to spool up more quickly than would otherwise be the case. However, as the supercharger is driven mechanically by the engine, it increases the load on the engine and thereby increases fuel consumption. Therefore, as soon as the turbocharger is effective, the supercharger may be deactivated.
Control strategies have been developed for operating a supercharger and turbocharger both sequentially and simultaneously, so as to provide the desired range of engine performance and engine response.
Superchargers may be driven directly from the vehicle engine, but this arrangement tends to be inflexible should the installation envelope change, for example because the engine is to be fitted to a different engine compartment. Accordingly, it has been proposed to drive the supercharger mechanically via the armature of the vehicle generator, which provides for different installation possibilities within an engine compartment.
In one arrangement the supercharger is mounted co-axially with the vehicle generator, which is in turn driven by the vehicle engine by a multi-vee belt which is mounted on a pulley which is coupled to the generator. The generator provides energy to the vehicle, and the armature of the generator is coupled mechanically to the supercharger via an epicyclic gearbox so as to provide a coaxial arrangement which increases the speed of the supercharger relative to the generator.
An epicyclic gearbox is composed of an outer gear ring or annulus, a plurality of inner planet gears referred to below simply as planets, and a central gear or sun. The sun is located at the centre of the annulus, with the planets located between, and meshing with, both the sun and the annulus. The planets are attached to a common planet carrier, which maintains their relative positions. If the planet carrier is rotated while the annulus is held stationary, the planets are caused to rotate around the inner surface of the annulus. As the planets rotate, this in turn causes the sun to rotate at a speed which is determined by the gearing ratio. Alternatively, the planet carrier may be held in position, in which case rotating the annulus causes the sun to rotate. In this way, the epicyclic gearbox provides a coaxial arrangement which generates the required speed.
The speed at which the vehicle engine operates is variable within a range of, for example, 500 to 7000 revolutions per minute (rpm). In the case where the planet carrier is held stationary, the arrangement described above produces a speed increase of around 30:1, through a combination of a 3:1 increase from the pulley, and a 10:1 increase from the epicyclic gearbox. Therefore, the expected operating range for the supercharger impeller in this arrangement is 15,000 to 210,000 rpm. However, in order to achieve optimal performance, it is desirable to maintain the speed of rotation of the supercharger impeller at a relatively constant level during operation, generally at around 120,000 rpm.
To this end, it has been proposed to couple the planet carrier of the epicyclic gearbox to an electric motor. By driving this motor forward or in reverse, as required, the speed of the supercharger can be adjusted to its optimal level. Power for the supercharger motor is provided from the vehicle generator, which is necessarily increased in size. This arrangement provides the benefit that the main work of driving the supercharger is performed by the vehicle engine, while the electric motor merely provides a fine-tuning action. The power requirement of the supercharger is such that it is impractical to attempt to use an electric motor in isolation to drive the supercharger in a vehicle environment.
If a supercharger is used in combination with a turbocharger, the supercharger is primarily active when the engine is operating at low speed, for example when the vehicle is pulling away. Once the engine speed is sufficient, the turbocharger takes over from the supercharger, as this saves on energy requirements. In the above-described arrangement, the supercharger is directly connected to the generator, and is thus constantly driven even when not being used. However it can be made to idle by, for example, opening the delivery side to atmosphere. This reduces the load on the supercharger impeller, and therefore in turn the energy which the supercharger draws when idling is reduced.
However, this arrangement retains the disadvantage that the supercharger is permanently connected to the generator armature, and thus absorbs some energy even whilst deactivated. It is well known that energy efficiency is a key concern in the design and manufacture of new vehicles, and therefore any measures which can be taken to reduce consumption are important.
Against this background, it is an aim of the invention to provide an arrangement that avoids idling of the supercharger, but maintains flexibility of installation.